This invention relates to internal combustion engines of the reciprocating piston-type, and more particularly to the provision of an improved balancing system designed to eliminate, or at least greatly reduce, vibration of the engine incident to its operation.
Internal combustion engines of the reciprocating piston-type require an arrangement for balancing the so-called primary vibration-causing inertial forces that result from reciprocation of their pistons and rotation of their crankshafts. These primary inertial forces are the result of the force required to start and stop the piston twice each revolution of the crankshaft. An engine balancing system is especially needed in small single cylinder and twin cylinder engines typically used for gardening tractors and riding mowers because if these inertial forces are not eliminated, or at least greatly reduced, objectional vibrations are transmitted to the operator of such machines, and to other parts of the machine.
As is well known, crankshaft-carried counterweights are incorporated in engines of this type in an attempt to reduce the primary vibration-causing forces. These crankshaft-carried counterweights are disposed 180.degree. opposite the crankpin of a piston so that when the crankshaft is rotated they produce centrifugal force having a vertical component that is in line with the piston axis and diametrically opposite the inertial force of the piston. In a condition of 100% overbalance these vertical force components substantially cancel out the primary vertical forces of the piston. However, the conventional crankshaft-carried counterweights also produce horizontal force components during rotation that are at right angles to the piston axis which at a condition of 100% overbalance produce excessive vibrations in the horizontal directions. As a result, engines of this type conventionally utilize crankshaft-carried counterweights that produce a condition of about 50% overbalance. Although not totally balancing either the vertical or the horizontal forces this condition of 50% overbalance produces tolerable vibrations for the system. It is desirable to provide a balancing system which would substantially balance or cancel out both the vertical and horizontal remaining primary vibration-causing forces. As noted above, the term "vertical" used in the present application refers to forces that are in line with the piston axis, and the term "horizontal" refers to forces that are normal to the piston axis.
Various balancing arrangements can be found in the prior art which attempt to balance or cancel out these primary forces, and yet meet the requirements of a practical and reliable engine balancing system which would not lead to extensive redesign of the engine. One such arrangement can be found in Harkness, U.S. Pat. No. 3,415,237 which is assigned to the assignee of the present invention. Harkness utilizes a pair of auxiliary counterweights mounted coaxially with the crankshaft on the exterior of the crankcase. The auxiliary counterweights are driven at crankshaft speed but in the opposite direction so as to coact with the crankshaft-carried counterweights to balance both the vertical and horizontal components of the primary inertial piston forces. However, the Harkness balancing system requires the use of extensive gear trains between the crankshaft and camshaft to drive the auxiliary counterweights. These gear trains are relatively expensive to manufacture and assemble, and further result in objectional noise during operation of the engine. There is also the problem of accommodating such gear trains in the limited space available for small compact engines of the type herein concerned. It is therefore desirable to provide an improved mechanism for driving the auxiliary counterweights of a Harkness-type balancing system.
In Nakamura et al, U.S. Pat. Nos. 4,125,036 and 4,074,589, there is disclosed a four cylinder engine having a pair of rotary auxiliary counterweights which rotate about axes that are spaced from the crankshaft and located within the engine block. These auxiliary counterweights are driven by a belt to rotate in opposite directions at twice the speed of rotation of the crankshaft. The balancing system of the two aforesaid Nakamura et al patents, however, is not concerned with balancing the primary inertial forces, but rather with balancing the secondary inertial forces which are particularly large on four cylinder engines. As is well known, these secondary forces are introduced by the geometry of the connecting rod and occur at twice the crankshaft speed. To that end, the auxiliary counterweights of the Nakamura et al patents rotate in opposite directions and at twice the speed of rotation of the crankshaft. Since the auxiliary counterweights rotate in opposite directions, the horizontal components of their centrifugal force vectors at any instant cancel each other out so as to avoid introducing any undesirable lateral forces into the system. However, the fact that these auxiliary counterweights rotate in opposite directions at twice the speed of the crankshaft prevents the auxiliary counterweights of the Nakamura et al patents from compensating for any primary forces. The teachings of the Nakamura et al patents thus lead away from the solution of the problem relating to balancing primary forces to which the present invention is specifically concerned.
Another balancing system is shown in German Pat. No. 1,935,121 which shows a four cylinder engine that incorporates a belt-driven compensating shaft that is spaced from and parallel to the crankshaft, and has a pair of auxiliary counterweights mounted thereon. The auxiliary counterweights are mounted outside the crankcase and rotate at the same speed as the crankshaft. The balancing system of the German patent, however, is not concerned with balancing the primary inertial forces, but only with balancing the torsional moments produced by multi-cylinder engines. Since the pistons of the four cylinder engine described in the German patent are offset from one another, they will produce an additional torque or moment that tends to cause engine vibration. This torque or moment may be present in some multi-cylinder piston engines, for example the four cylinder engine of the aforesaid German patent and in-line, uneven-firing twin cylinder engines. Such vibration-producing moments, however, are not present in single cylinder and in-line in phase twin cylinder piston engines of which the present invention is concerned. The auxiliary counterweights mounted on the compensating shaft as disclosed in the aforesaid German patent are thus designed to substantially balance or cancel this added torque or moment by producing another torque or moment in the opposite direction. If such a system were incorporated with single or in-line even-firing twin cylinder engines, it would not balance the primary inertial forces created by such an engine, but in fact would create an additional undesirable torque or moment instead. The teachings of the German patent thus lead away from a solution of balancing primary inertial forces to which the present invention is specifically directed.
The series 700 single cylinder diesel engine produced by Lombardini Motors, an Italian engine manufacturer, incorporates yet another type of balancing system. The series 700 engine includes a single auxiliary counterweight that is mounted on a shaft spaced from and parallel to the crankshaft. The auxiliary counterweight is rotated at the same speed as the crankshaft by means of a gear train located between its shaft and the crankshaft. The auxiliary counterweight, however, is mounted inside the engine block which creates interior design problems for the engine. Also, since there is only one auxiliary counterweight and it is offset from the crankshaft axis, a torque or moment will be created which will cause the engine to vibrate with a rocking motion. Thus, if the Lombardini balancing system was incorporated with single cylinder or in-line even-firing twin cylinder piston engines, it would create an additional undesirable torque or moment. The Lombardini system therefore does not solve the problem of balancing primary inertial forces to which the present invention is specifically directed.